There has been a desire in recent years for rapid practical application of clean, highly efficient fuel cell systems in order to solve problems relating to energy and the environment.
In the case of presuming use of a polymer electrolyte fuel cell that operates at low temperatures, the fuel that is used is hydrogen. Consequently, after carrying out a desulfurization procedure as necessary, it is necessary to catalytically reform methane, natural gas consisting mainly of methane, or other hydrocarbons (such as propane gas, liquefied petroleum gas, gasoline, diesel fuel, petroleum or kerosene) for conversion into a product gas containing hydrogen. This process of producing hydrogen by reforming is one of the key steps for increasing the power generation efficiency of fuel cells.
In contrast, solid oxide fuel cells and molten carbonate fuel cells, which operate at moderate and high temperatures, are characterized by allowing hydrocarbons to be used directly. However, in this case as well, a process for reforming the hydrocarbons is frequently still necessary for reasons such as tendency of hydrocarbons to carbon precipitation and the use of hydrogen rather than hydrocarbons facilitating power generation.
Examples of methods for producing hydrogen by reforming hydrocarbons include methods that use a steam reforming reaction, carbonic acid gas reforming reaction and direct heat supply reforming reaction. Among these production methods, the direct heat supply reforming reaction combines an exothermic reaction in the form of perfect combustion with an endothermic reaction in the form of a steam reforming reaction or carbonic acid gas reforming reaction, and consists of, for example, carrying out complete combustion in the first half of the catalyst layer of the reactor, after which the heat generation thereby propagates to the latter half of the catalyst layer of the reactor, thereby promoting the reforming reaction in the form of an endothermic reaction. Consequently, in comparison with the steam reforming reaction and carbonic acid gas reforming reaction that consist of endothermic reactions only, direct heat supply reforming is superior from the viewpoint of reducing the external supply of heat, and the hydrogen formation rate is also extremely fast. On the basis of this background, development is proceeding on hydrogen production catalysts that use direct heat supply reforming of hydrocarbons.
Hydrocarbon reforming reactions have already been industrialized, and in particular steam reforming is employed in a wide range of applications, with the product gas in the form of a synthesis gas (CO/H2) being used in applications such as Fischer-Tropsch synthesis. However, in contrast to operation being continued for long periods of time in conventional applications, there are cases in which the device is frequently started and stopped when assuming application to on-site reforming fuel cell processes. For example, devices can be assumed to only be used in the home during daylight hours or only used in vehicles during driving, while being stopped at all other times. In addition, in the case of off-site reforming such as hydrogen stations as well, the device is only operated during the day. Although it is desirable that electrical power or other form of external energy not be used during starting of the reformer, even in the case of direct heat supply reforming that demonstrates superior hydrogen production even when starting production from low temperatures, it is still necessary to heat the catalyst layer to 200° C. (Patent Document 1 and Non-Patent Document 1). Consequently, electrical power or other external energy must be used when starting the reformer, thereby resulting in problems regarding use in areas not supplied with power and use during disasters. This has also been a considerable problem in terms of realizing fuel cell vehicles loaded with gasoline, gas oil or other liquid fuel.
Patent Document 1: JP 2006-190605 A
Non-Patent Document 1: Kenneth A. Williams and Lanny D. Schmidt, Applied Catalysis A: General 299 (2006) 30-45